It is well-known to remove sulphur compounds such as hydrogen sulphide, sulphur oxides, carbon disulphide and lower alkyl mercaptans from gaseous flows by means of scrubbing the gaseous stream in an absorption tower with an aqueous washing solution, such as for example a solution of sodium or potassium carbonate. Thus, a purified gaseous stream and a washing solution loaded with sulphide is obtained, optionally after reduction of dissolved sulphur oxides into sulphide.
In WO92/10270 a process is disclosed wherein an aqueous solution containing sulphide obtained from scrubbing a sulphur-containing gaseous effluent is subjected to sulphur-oxidising bacteria in the presence of oxygen in a reactor to oxidise the sulphide to elemental sulphur and hydroxide.
In WO94/29227 a process is disclosed for oxidation of sulphide to elemental sulphur with sulphide-oxidising bacteria in an airlift loop reactor wherein a vertical circulation is maintained by means of an oxygen-containing gas flow.
In a process for the biological oxidation of sulphide, it is important to minimise the undesired production of sulphate and to maximise the desired production of elemental sulphur. It is known that the formation of elemental sulphur over sulphate can be promoted by controlling the oxygen supply.
In WO98/04503 a process is disclosed for the biological treatment of a spent caustic solution comprising sulphides in an aerobic reactor containing sulphur-oxidising bacteria wherein the redox potential in the reactor is controlled. In the process of WO98/04503, the sulphide oxidising reaction is controlled, i.e. elemental sulphur formation is promoted over sulphate formation, by adjusting the redox potential of the medium of oxidation at a value below −300 mV.
Outside the field of bacterial treatment of aqueous solutions, WO01/27042 is specifically concerned with the treatment of process fluids containing solid particles. In WO01/27042 an apparatus is described containing a treatment chamber having at least two interior regions wherein gas bubble inlets and fluid inlet and outlet ducts are arranged such that the process fluid is caused to follow circulating (spiraling) paths in opposite (clockwise vs. anticlockwise) directions along the length of chamber. The circulation-driving bubble inlets are arranged in a plurality of rows so as to create a “curtain” of bubbles between two regions of opposite circulation, such that the process fluid must pass across the curtain of bubbles in its transit through the vessel, whereby any relatively dense solid particles will, on passing through the bubble curtain, experience a significant reduction in buoyancy thereby falling into a solids-collection region arranged between the bubble inlet rows.
There is a need in the art to improve processes for oxidation of sulphide by using sulphide-oxidising bacteria by further controlling the oxidation reaction, in particular to avoid undesired abiotic reactions such as thiosulphate formation due to locally high sulphide concentrations in the reactor.